1. Field of the Invention
The present invention generally relates to lithography, and more particularly to an anti-reflective coating for optical elements.
2. Background Art
Lithography is widely recognized as a key process in manufacturing integrated circuits (ICs) as well as other devices and/or structures. A lithographic apparatus is a machine, used during lithography, which applies a desired pattern onto a substrate, such as onto a target portion of the substrate. During manufacture of ICs with a lithographic apparatus, a patterning device (which is alternatively referred to as a mask or a reticle) generates a circuit pattern to be formed on an individual layer in an IC. This pattern may be transferred onto the target portion (e.g., comprising part of, one, or several dies) on the substrate (e.g., a silicon wafer). Transfer of the pattern is typically via imaging onto a layer of radiation-sensitive material (e.g., resist) provided on the substrate. In general, a single substrate contains a plurality of target portions that are successively patterned. Manufacturing different layers of the IC often requires imaging different patterns on different layers with different reticles. Therefore, reticles must be changed during the lithographic process.
Lithographic apparatus, and patterning devices included therein, often include one or more mirrors to reflect and redirect beams of radiation. For example, such apparatus often include micro-electromechanical systems (MEMS) that include arrays of discrete mirrors. Mirrors in such systems are formed by depositing a highly-reflective coating (e.g., layers of aluminum, aluminum oxide, silicon oxide and various metal fluorides) onto a portion of a surface of a silicon substrate to reflect incident radiation in specified manner.
However, uncoated silicon can reflect up to 60% of incident radiation at deep ultraviolet (DUV) wavelengths (as compared to less than 5% reflection from fused silica substrates or calcium fluoride (CaF2) substrates over a comparable range of wavelengths). Hence, light reflected from a portion of the substrate that is intentionally not coated by the highly-reflective coating can be comparable in intensity to that of light reflected from the portions coated with the highly-reflective coating. Thus, a bare silicon surface can scatter reflected light, which can interfere with radiation reflected by the portions coated with highly-reflective coating.
As such, an anti-reflective coating is often applied to those exposed portions of the surface of the silicon substrate to substantially reduce or eliminate undesirable reflections and scattering of incident radiation. However, unlike existing mirror systems for use in lithographic apparatus that incorporate fused silica or calcium fluoride substrates, efficient, single-layer antireflective coatings are generally not possible for MEMS like optical devices that incorporate silicon substrates. Methods and systems are needed to overcome the above mentioned deficiencies.